One example of a radial turbomachine is a multistage single-shaft radial compressor, the individual stages of which have, as components for guiding the flow of process gas, an impeller, which is driven by a shaft, a radial diffuser, a deflecting channel and a return flow channel. The process gas flows first through the impeller, wherein the process gas enters the impeller in the axial direction and leaves the impeller in the radially outward direction. An increase in the static pressure takes place in the diffuser, which connects radially to the impeller and through which the process gas flows from the inside outward. In the multistage radial compressor, the process gas flows through a multiplicity of radial compressor stages, which are arranged axially in series on the shaft and each have the impeller and the diffuser. In each of the radial compressor stages, the gas flow is furthermore guided back radially in the direction of the shaft downstream of the diffuser in order to enter the impeller of a subsequent radial compressor stage. The return flow is accomplished by means of a deflecting channel, which deflects the flow direction of the process gas from radially outward to radially inward. Connected to the deflecting channel on the downstream side is a return flow channel, which extends in the radial direction in order to guide the flow of process gas to the inlet of the impeller arranged downstream. After flowing through the final radial compressor stage, the process gas flows into a spiral housing which connects to the final radial compressor stage and to which a radial compressor stub is connected, through which the process gas flows off. The practice of removing part of the process gas from the main flow in order to obtain cooling gas, e.g. to cool a motor or to cool magnetic bearings, is known.
However, among the disadvantages of removing the partial gas stream from the main flow is that, in many cases, the quality of the partial gas stream is inadequate in terms of purity requirements since the partial gas stream may have an excessive moisture content, for example. This can lead to damage to the components that come into contact with the partial gas stream and, as a result, the maintenance cycles of the radial turbocompressor are short.
FR 1 326 166 A has disclosed a boundary layer extraction system for a compressor of the type in question which is suitable in principle for extracting clean process gas.